* src/reader.c (token_translations_init): 256 is now the default

* src/reader.c (token_translations_init): 256 is now the default
value for the error token, i.e., it will be assigned another
number if the user assigned 256 to one of her tokens.
(reader): Don't force 256 to error.
* doc/bison.texinfo (Symbols): Adjust.
* tests/torture.at (AT_DATA_HORIZONTAL_GRAMMAR)
(AT_DATA_TRIANGULAR_GRAMMAR): Number the tokens as 1, 2, 3
etc. instead of 10, 20, 30 (which was used to `jump' over error
(256) and undefined (2)).